This invention relates generally to systems and methods relating to centrifuges. More specifically, this invention is an improved control system for detecting imbalances in centrifuges.
Centrifuge technology presents unique design criteria wherein precision control of the rotational operation of the centrifuge is required. Most centrifuge technology is used for biological and chemical experimental research, which uses centrifugation as their primary tool to achieve component separation and perform experimental assays. These types of centrifuges carry light payloads. However, another class of centrifuges exists for carrying larger payloads, ranging in excess of 200 lbs. Centrifuges of this type are used to assess the effects of stress on its payload.
When a centrifuge is used, the centrifuge rotor is driven to extremely high rotational speeds in order to generate the centrifugal field required for research use. A large amount of kinetic energy is built up from the high rotational speeds of the motor. If the kinetic energy is uncontrollably released it can lead to destructive explosion of the centrifuge and injury or damage to its surrounding environment, including the human operator. Centrifuge rotors typically can fail if the rotor is run in excess of the speed designed for its safe operation. The slightest imbalance of the rotor or payload, which it carries, can cause catastrophic failure.
Furthermore, even the slightest imbalance of the rotor or load being carried may grow to larger imbalances, and associated forces, as the rotor speed and centrifugal field increase. Often, these imbalances do not arise until the rotor has achieved very high speeds. The dynamic effect of any imbalancing forces causes complicated movement of the shaft upon which the rotor is suspended, such as dangerous whirls and gyrations. Thus, many systems have been developed to detect such imbalances and are described herein.
The following references generally describe systems and methods for detecting imbalances in centrifugal devices. The references can generally be divided into two groups. The first group either uses a sensor to detect a change in distance between a reference position and a position of the rotor, thereby generating a distance detection signal, or a mechanical switch to shut down the system when it is out of balance.
U.S. Pat. No. 3,422,957 to Fosler describes an unbalanced sensing switch assembly of this first group for centrifugal machines. The system comprises a centrifuge basket coupled to a drive unit by a shaft. A bump switch having a micro-switch component is secured to the outer portion of the shaft. The bump switch assembly is operates to detect unbalances in the load and to shift the drive unit to lower speeds to prevent the unwanted vibrations.
U.S. Pat. No. 4,099,667 to Uchida describes an apparatus for preventing vibration in a centrifugal separator comprising an upright electric motor supported by a resilient member from a machine casing. A mercury type, vibration sensitive, element is used to sense vibrations and open an electric power circuit of the motor.
U.S. Pat. No. 4,214,179 to Jacobson describes a rotor unbalance detector for a centrifuge. The device includes a rotatable electrically conducting ring surrounding a shaft. Washers and an o-ring insulate the electrically conducting ring from the shaft. The shaft is connected to a rotor and chamber. When the rotor chamber becomes unbalanced, the shaft is forced to rotate off its natural axis of rotation. If the axis of rotation differs by a sufficient amount, the shaft will contact the conductive ring, de-energizing the power supplied to the motor and causing it to stop rotating.
U.S. Pat. No. 5,160,876 to Niinai describes a system and method for precisely detecting the unbalance of a rotating body without being adversely affected by external disturbances. Specifically, a rotor is connected to a drive shaft. A displacement sensor is placed in proximity of the drive shaft for detecting the amount of imbalance of the rotor in terms of the vibration amplitude of the rotor. The sensor is connected to an electronic circuit that is in turn connected to a control unit. The control unit contains a microprocessor that performs an arithmetic processing operation according to an algorithm stored therein for calculating a control signal based on the vibration amplitude, derived from the vibration sensor, and time.
The second group of references pertains to unbalance detection systems utilizing an electronic sensor for measuring vibrations or physical stress on the system.
U.S. Pat. No. 54,879,279 to Berger is directed to a centrifugal separator apparatus having a vibration sensor. A dual mode vibration sensor is located radially outward, mounted to the frame of the centrifuge, from a shaft used for rotating a bowl. The vibration sensor is for detecting radial vibrations of the bowl during operation of the centrifuge. Upon the vibration sensor sensing radial vibrations above a first predetermined threshold or a second predetermined threshold, a signal is sent to a controller that activates a D.C. brake or frequency inverter to stop the rotation of the bowl.
Finally, U.S. Pat. No. 5,857,955 to Phillips is directed to a centrifugal control system utilizing a control computer program and a variety of sensors. The computer has several input terminals, two of which are connected to the drive units of the centrifuge. Two output terminals of the computer are used for sending signals to the drive units and to vary the frequency and voltage applied to the AC motors. The variation in the frequency and voltage accordingly varies the rotation and torque applied to the drive shaft. A vibration sensor connected to the outer bowl of the centrifuge sends signals to the computer regarding vibrations associated with the centrifuge. The computer responds to excess vibrations of the centrifuge by generating an output signal causing the drive units to turn off the motors, shutting down the centrifuge.
The above references describe the many attempts to provide an imbalance detection system. While some may work for small-scale centrifuigal systems, adequate detection and control for large centrifuge systems is not possible using the above described systems. Although large centrifuge systems encounter many of the problems of their small-scale counterparts, they also face unique problems that are not addressed by the above references. For example, largescale centrifuges carry payloads in excess of 200 lbs. traveling at speeds in excess of 750 rpms. At these speeds, the payloads are subject to forces in excess of 300 g""s, where a xe2x80x9cgxe2x80x9d is the force of gravity on the object, which is created by the spinning motion of the centrifuge. Furthermore, certain types of testing require instant ramp-ups and -downs of speed of the centrifuge, which cause vibrations resembling imbalances in the payload. The ramp-ups and -downs also place unusual stresses on the centrifuge.
Thus, what is clearly needed to insure centrifuge safety and sample integrity is a system and method of controlling a centrifuge that distinguishes between normal operational vibrations, including vibrations from sudden ramp-ups and -downs, from vibrations caused by true load unbalances. More specifically, a system and method that can detect load unbalances as low as 5 lbs. in a 200-lb. payload, which also allows the user to monitor the imbalance as the centrifuge is operating.
This invention relates to a system and method for detecting and controlling payload imbalances in centrifuges. In one embodiment, the system consists of an accelerometer sensor attached to a centrifuge enclosure. The system uses an accelerometer in the 0.01-0.02g range for sensing vibrations. The output signal of the sensor is an electrical charge in the form of a sine wave with an amplitude and frequency. The charge is passed through a pre-amplifier to intensify the signal and convert it to a voltage signal. The amplified signal is fed through a low pass filter for removing extraneous vibrations associated with general operation of the centrifuge. The filtered signal is passed through an A/D converter before being fed into a processor for analysis by an algorithm. The algorithm interprets the amplitude and frequency associated with the signal. Once an amplitude threshold has been exceeded, the software algorithm begins to count cycles during a predetermined time period. If a given number of complete cycles occurs during the threshold time period, the system shuts down the centrifuge. The number of cycles over time helps to distinguish between true unbalances and those vibrations caused by sudden ramp-up or -down of the centrifuge, which are transient vibrations. In the event of a true imbalance, typically greater than 5 lbs., the computer sends a signal to a relay or other circuit causing the centrifuge to automatically shutdown. During operation of the centrifuge, the computer automatically displays the relative imbalance value so the operator can immediately tell the condition of the system even before a 5-lb. unbalance occurs.
Some objects of this invention are to:
provide an imbalance detection system that can measure payload imbalances in centrifuges;
provide an imbalance detection system that can differentiate between normal vibrations, due to sudden ramp-ups and downs, from vibrations due to payload imbalances; and
provide a method to detect imbalances and differentiate between normal vibrations, due to ramp-ups and downs, from vibrations due to payload imbalances.